1. Field of the Invention
The present invention generally concerns monolithic multiple buried-substrate capacitors, particularly miniature buried-substrate capacitors such as are useful in small electronic circuits, including in hearing aids that are inserted in the external acoustic meatus, or ear canal.
The present invention particularly concerns the physical--mechanical and thermal--mounting of electrical circuits and components on and to monolithic multiple buried-substrate capacitors, and the simultaneous electrical connection--realized other than by the physical mounting--of such circuits and components to the buried capacitors that are located within the monolith. The mounted and electrically connected electrical circuits and components are commonly monolithic integrated circuit transistor receivers and/or amplifiers particularly as are used in hearing aids, and the electrically-connected buried capacitors particularly serve as a filter.
2. Description of the Prior Art
2.1 The Structure of Monolithic Multiple Buried-Substrate Capacitors
Monolithic buried-substrate, and multiple buried-substrate, capacitors are sometimes identified with the two words "substrate" and "capacitor" reversed, and are called Buried Capacitor Substrates, or BCS. Howsoever called, BCS accord significant size reduction to microelectronic applications. A BCS integrates capacitors, resistors and traces into a thin multi-layer substrate, which can be joined with IC devices. Volumetric reductions of 50 to 75% are possible because the BCS eliminates the air gaps between passive components and replaces the alumina substrate of traditional hybrids. This accords the freedom to mount one or more ICs onto the BCS (or perhaps even one on each side) and then attach the device to a ribbon lead, larger hybrid, or Multi Chip Module. The BCS is also compatible with flip chip IC Designs, giving the most dense hybrids presently possible, circa 1995.
Substrate density and parasitic management are the technological keys to BCS Miniaturization. A BCS desirably uses the finest possible spacing for electrical connections around the edge of the chip (020"), offering the greatest number of connections between the IC and buried capacitors. Inside the chip, multi-layer technologies are used. Stray capacitance within the chip is controlled by (i) use of high and low K dielectrics, and (ii) the unique castellation cutting technique taught in the related U.S. Pat. No. 5,367,430 for a MONOLITHIC MULTIPLE CAPACITOR. These techniques combine to give low noise between internal capacitors and maintain parasitic capacitance at low levels.
The surface metallization of the BCS is made from co-fired material. It is commonly available in either an as-fired solderable, a nickel barrier solderable, or a gold over nickel wire bondable system. This allows the chip to be used in solder applications, various types of wire bonding, or flip-chip die attach. The chip can be solder attached to either a larger hybrid, a Multi Chip Module, or a flexible ribbon cable. IC's and discrete components can be attached to one or other sides. The surface traces are typically printed with 5 mil lines and 5 mil spaces in any pattern required to fit the application.
A series of castellations along the side of the BCS provide electrical connection from capacitors within the chip to the top and bottom surfaces. These castellations consist of metallized pads separated by 6 to 12 mil deep air gaps. A pitch 0.020 inches between castellations is possible, while still retaining excellent solder reflow characteristics. These castellation are, again, the subject of related U.S. Pat. No. 5,367,430 for a MONOLITHIC MULTIPLE CAPACITOR.
A soldered connection to a castellation can provide a variety of electrical connections: directly to the IC, through a passive component, then to the IC; or to the IC with a capacitor to ground. In cases where an IC on each side of the BCS requires many I/O connections, a series of castellations can be made with 5 mil pads and 5 mil spaces.
A series of castellations along the side of BCS provide electrical connection from capacitors within the body of the BCS to its top and bottom surfaces. These castellations consist of metallized pads separated by 6 to 12 mil deep air gaps. A pitch 0.020 inches between castellations is possible, while still retaining excellent solder reflow characteristics.
BCS can be produced in a variety of sizes. A typical minimum size is 0.070 by 0.070 inches. The size of a BCS will usually be chosen based on capacitance values desired, the voltage rating, and internal space needed to control stray capacitance. Any sizes up to 1" by 1" are possible, with maximum capacitance values near 10 uF: again, capacitance value achievable depends on voltage rating required.
Because BCS contain non-symmetric buried plates, variation in surface flatness can be expected. Production techniques allow three controls of this irregularity. BCS can be made so that the top surface is flat and the bottom surface contains all the irregularity. It can be made so that the bottom surface is flat and the top surface contains all the irregularity. Finally, a BCS can be made so that the irregularity is averaged on both sides, so that it is equally absorbed by the top and bottom surfaces.
Internal conductor traces can be used to connect castellations on one area of the chip to the other. Usually this is needed when internal capacitor arrangements make conventional layouts impossible, or to accommodate existing IC pad locations. The layer of traces can be put in at any level within the chip, depending on what is optimal for reducing stray capacitance.
Various designs of the internal plates of a BCS are possible. Capacitance value depends on the active area of each capacitor and the number of layers used.
It is possible to design one or more capacitors against an internal ground plane. By changing the position of connecting tabs, almost any connection configuration can be achieved.
Shielded capacitors are created by having a series of capacitor plates which are surrounded by two ground planes. This shielding can occur above and below the capacitor or along the edge of the device.
Coupling capacitors can be designed using a stack of individual opposing plates. Again, capacitance value depends on the number of layers and the active area of each chip.
In a typical BCS device, capacitors are stacked in different levels within the chip; one or more capacitors are built on each level, and each capacitor tabs out to a different castellation. Typically one castellation would connect to all internal ground planes.
Stray capacitance between different internal capacitors is controlled by varying the distance between the capacitors on the same level or by varying the layer thickness between levels.
BESS are commonly made from any of NPO, X7R, and Z5U dielectrics. For ease of designing a substrate, the dielectrics can be thought of as capacitance achieved per unit area, given a normalized dielectric thickness.
For an area 0.1 inches on a side, at a dielectric thickness of 0.001 inches, capacitance values are as follows:
______________________________________ Dielectric Cap per .01 in square @ 1.0 mil thick ______________________________________ NPO 312.7 pF X7R 7417 pF Z5U 26.477 pF ______________________________________
2.2 The Use of, and Previous Electrical Connection to, Monolithic Multiple Buried-Substrate Capacitors
Monolithic multiple buried-substrate capacitors contain, just as their name indicates, multiple capacitors within a single, monolithic, body. Monolithic multiple buried-substrate capacitors are typically electrically connected to, and useful in combination with, small, integrated, electronic devices, including amplifiers and receivers of hearing aids that are inserted in the external acoustic meatus, or ear canal. The substantial purpose of putting multiple buried-substrate capacitors within a single monolith is to save volume; otherwise a number of separate chip capacitors could be conventionally employed.
Electrical connections to multiple buried-substrate capacitors are, however, multiplied by the number of such separate connections to be made, and aggravated by the typical microminiature size of a multiple buried-substrate capacitor--typically as small as 0.070".times.0.070".times.0.020" thickness--and by the commensurate microminiature size to the electronic devices to which it is connected.
The connected electronic devices--amplifiers and receivers and the like requiring connection to external capacitors for filtering purposes--are typically provided with leads. It has been known to place holes, or bores, in the body of a multiple buried-substrate capacitor--including by process of laser drilling--and to then attempt to place the leads in the bores to attempt down-hole electrical connection with selected electrodes of the buried capacitors. This effort has essentially come to naught. In the first place, the hole placement, by laser drilling or otherwise, tends to displace the conductive material of the electrodes that are penetrated down hole, and to leave an insufficient amount of this material at the walls of the bores at, and only at, the regions of the exposed down-hole electrodes so as to permit electrical connection to be reliably made. Moreover, and equally importantly, it is all but impossible to wick solder into the typically small holes, the access to which may also be, should the electronic device be mounted flush as is desired, impeded.
Accordingly, the typical electrical connection of leaded electronic devices--amplifiers and receivers and the like--to the capacitors that are within the body of a multiple buried-substrate capacitor is by (i) bringing the electrodes of the capacitors to some localized, pad, region of the surface of the multiple buried-substrate capacitor, and then (ii) soldering the leads of the electronic devices to these pads. To say that this work is delicate and painstaking, and therefore expensive, is an understatement. It is typically performed by dexterous women viewing their soldering though microscopes. Difficulties in electrically connecting, circa 1995, one microminiature component to another is one reason that very, very small electronic items such as hearing aids that fit within the external acoustic meatus, or ear canal tend to be expensive, costing several hundreds and even thousands of dollars U.S. circa 1995.
One known way of reliably efficiently electrically small things is by reflow soldering. However, a microminiature multiple buried-substrate capacitor will generally not stay sufficiently precisely located to a microminiature electronic device, and vice versa, so as to reliably permit reflow soldering to transpire.
According to these difficulties, it would be useful if an improved electrical connection between at least a microminiature multiple buried-substrate capacitor and another microminiature electronic device, if not between microminiature electronic components and devices in general, could be developed.
2.3 Filters
The use of capacitors in filters is well understood. A major purpose of electrically connecting microminiature electronic devices such as audio amplifiers and receivers and the like (not necessarily in hearing aids, but also in, for example, telephones and radios) to microminiature multiple buried-substrate capacitors is to create capacitor filters.
It has been noticed by the inventors of the present invention that the capacitor filters so created are, while typically optimal for filtering one type of noise, not always optimal for filtering all the noise that is present at the location within the electronic circuit where the filter appears.
In particular, the most common, and pervasive, type on noise filter is directed to removing a relatively lower-frequency, less than 100 kHZ, electrical noise that chiefly results from and arises in a modern-era switching-type power supply. Because reactance is a function of frequency, the high switching frequencies of switching-type power supplies permit miniaturization of the reactive components of these supplies, and of an entire supply. No one is suggesting that these switching-type power supplies should be supplanted or eliminated, it is simply the case that, at least in microminiature form, they are not noiseless.
Furthermore, and this time for reasons of communications theory, and of communications density and capacity, digital switching is very often occurring, and noise is attendantly being developed, at much higher frequencies of, for example, 100 Mhz. For example, and as an arbitrary example, consider the new GMS mobile telephones now in use in Europe. A 1 Ghz radio carrier is pulse modulated at, typically [100 Mhz?], at amplitudes up to 3 volts. The field strength at the microphone of a mobile telephone may easily approach 1100 volt/meter. This varying electrical, field is picked up in the leads of the microphone, and constitutes noise to the microphone audio amplifier.
It is know to construct filters of two, or more, capacitors to filter out noise in multiple frequency bands simultaneously. However, the problems of so doing in the microminiature, difficult-to-connect, realm of multiple buried-substrate capacitors are daunting. To the best knowledge of the inventors, it has not only not been previously attempted to construct multiple-capacitor filters at this size scale, but it has not even been recognized to be possible to do so.
2.4 Through-Holes
Through-holes have previously been placed in ceramic capacitors for purposes of mounting electrical components to the capacitors (equivalently; mounting the capacitors to electrical components) by wire leads that extend within the holes. The holes have heretofore been made by punching or by drilling, and are quite large (relative to the wire) as best suits the wicking of solder or conductive adhesive into the hole so that a wire inserted within the hole may be electrically connected in downhole regions to the plates of buried-substrate capacitors that are intentionally brought to the boundaries of the holes.
Two limitations are inherent in this scheme. First, because solder or conductive adhesive must be wicked into the hole that also contains the wire, clearance room must be provided at the top of the hole, making that the wired-leaded component cannot be mounted tight against the ceramic capacitor with holes. The necessary separation between holed capacitor and wire-leaded component not only results in reduced packing density, but also in reduced mechanical strength and tolerance to shock and vibration.
Second, it is known to drill laser holes, which can be quite small, in microminiature items. For purposes of putting multiple small holes in ceramic multiple capacitors, particularly, laser hole drilling would seemingly be of interest. However, a laser drilling of any ceramic capacitor, including a ceramic multiple capacitor, tends to cause destructive de-lamination, and or ablation of material, where the laser beam penetrates the conductive plates of the buried-substrate capacitor(s) in its (their) down-hole positions. The result of laser drilling a ceramic capacitor is a hole which, even if having satisfactory physical quality (which is difficult, and dubious), has insufficiently reliable continuity to electrodes exposed in down-hole so as to support reliable soldered (or conductively adhered) connection(s) of, wire(s) placed within the holes.